What happens to phosphoric acid molecules at high temperatures to form larger phosphorus-containing acids?

When phosphoric acid molecules are subjected to high temperatures, they interact and undergo a process of polymerization that typically involves the loss of water molecules.12

This reaction leads to the formation of larger phosphorus-containing acids through the creation of P–O–P bonds. Key examples include:12

• Diphosphoric (pyrophosphoric) acid: Formed from two molecules of phosphoric acid with the loss of one molecule of water, resulting in the formula ${\text{H}}_4{\text{P}}_2{\text{O}}_7$.1
• Polyphosphoric acids: A homologous series of long-chain molecules with the general formula $\text{H}({\text{HPO}}_3{)}_n\text{OH}$ (where $n\ge 2$).1
• Metaphosphoric acids: A class of polymeric acids with the general formula $({\text{HPO}}_3{)}_n$, which are characterized by cyclic molecular structures.1

The term metaphosphoric acid is also used to describe a viscous, sticky substance consisting of a mixture of both long-chain and ring forms of these polymers.1